1. Field of the Invention
The present invention relates to a linear motor of the moving-magnet type and a tool moving device provided with the linear motor. Particularly, it relates to a linear motor in which a magnet yoke is made to be thinner for a lighter moving part, and a tool moving device provided with such a linear motor.
2. Discussion of the Related Art
Generally, as described in, for example, U.S. Pat. No. 5,357,158 (equivalent of Japanese unexamined published patent application No. 5-227729), a linear motor is composed of a primary side and a secondary side. The primary side includes a rectangular plate-like coil yoke 25 provided on a rail track 11 to extend over the entire length of the rail track and a plurality of armature coils 26 arranged on the coil yoke to make a line, whereas the secondary side includes a magnet yoke 32 secured to a lower surface of a sliding unit 13 slidable on the rail track and a rectangular plate-like field magnet 33 secured to a lower surface of the magnet yoke to face the armature coils 26.
The field magnet 33 has been magnetized to arrange a plurality of N and S magnetic poles in an alternate fashion in the sliding direction of the sliding unit. With predetermined electric current applied to the field coils, a propelling force according to Fleming's left-hand rule is generated between the primary side and the secondary side, whereby the sliding unit is slidden on the rail track.
In the linear motor of this kind, the propelling force acting on a movable body (like the aforementioned sliding unit) is determined in dependence on the direction and the magnitude of the electric current supplied to the field coils. In the prior art linear motor described in the aforementioned patent, all the magnetic flux depending on the electric current supplied to the field coils passes the magnet yoke, and hence, the magnet yoke has to have the minimum cross-section which does not cause magnetic saturation of the magnetic flux to take place. That is, the magnet yoke should have the thickness which depends on the magnitude of the magnetic flux passing therethrough, so that the linear motor is increased in dimension and weight. In addition, in order to increase the power of the linear motor, a plurality of magnet members have to be arranged, and therefore, in a liner motor of the moving-magnet type, a problem arises in that the linear motor is increased in dimension and weight more and more.